1. Technical Field
The disclosure relates to a method of preparing water-insoluble fibers from an extremely hydrophilic polypeptide, which is very soluble in water. More particularly, the disclosure relates to a water-insoluble polyglutamic acid fibers and a preparation method thereof.
2. Description of Related Art
There are two kinds of commonly used materials with high water-absorption ability. One is based on acrylic acid, and the other is based on carbohydrate. For the one based on acrylic acid, the material is low-cost and has significant water-absorption ability, but it is not biodegradable and will therefore cause environmental problem. For the one based on carbohydrate, such as starch, chitin, sodium alginate, and carboxymethyl cellulose (CMC), the material is low-cost and biodegradable, but its water-absorption ability is limited. Therefore, it is needed to develop a material that has significant water-absorption ability and biodegradability.
It is well known that polyglutamic acid (PGA) has high water-absorption ability and high biodegradability, and PGA is also very soluble in water. Therefore, PGA can be easily dissolved once if PGA gets in touch with water or even only exposes to water vapor in the air. This is why PGA without any treatment cannot be used as water-absorption material and hence cannot be spun into fibers. Conventionally, PGA is cross-linked to have its conformation maintained and also to avoid being dissolved.
The methods of cross-linking PGA include physical methods and chemical methods. In physical methods, e.g. JP Publication No. 6-322358, the PGA is cross-linked by γ-ray to produce hydrogel with high water-absorption ability. However, the γ-ray equipment is complicated and expensive. Therefore, this method is not suitable for industry.
In chemical methods, a cross-linking agent is used to perform cross-linking reaction on PGA. Conventional cross-linking agents contain functional groups of dialdehyde, diamine, or diepoxide. For example, JP Publication No. 11-343339 discloses a method of isolating a high concentration γ-PGA from a culture broth, and using the isolated γ-PGA as the starting material for the cross-linking reaction with a diepoxy compound to obtain a biodegradable, water absorbable hydrogel. In U.S. Pat. No. 6,998,367, a cross-linking agent having dialdehyde, diamine, or diepoxide functional groups and a metal ion were used to cross-link PGA to produce water absorbable material that is water-insoluble. In U.S. Pat. No. 7,125,960, glutaraldehyde, ethylene glycol diglycidyl ether, and carbodiimide were used to cross-link PGA to obtain water absorbable gel.
The methods above have disclosed that water-insoluble PGA gel can be obtained by cross-linking technique, but the water-absorption ability of PGA was largely decreased. Hence, the biomedical applications of PGA are limited. Accordingly, if PGA can be spun into water-insoluble fibers, the above mentioned problem can be solved. However, the unmodified PGA is very soluble in water, and hence conventional spinning methods cannot be used to spin PGA fibers. Although the available techniques are able to produce water-insoluble PGA, only formation of PGA gel was disclosed. Nothing about how to form PGA fibers by spinning technique was disclosed. Moreover, the PGA gel does not provide sufficient flowability and hence cannot be spun by conventional spinning methods.